An electromagnetic component such as an inductor and a motor generally has a structural unit made by forming an electrical conductor coil around a magnetic core (a core). In recent years, the use of the powder magnetic core as a magnetic core (a core) has been studied. The powder magnetic core is produced by compacting a soft magnetic powder and has isotropic magnetic properties. Therefore, the powder magnetic core enables a three-dimensional magnetic circuit to be designed and can contribute to production of smaller and lighter electromagnetic components.
Examples of the magnetic properties that a magnetic material exhibits when the magnetic material is magnetized include an iron loss, a magnetic flux density, a coercive force, and frequency characteristics. Examples of the magnetic properties that are important for the powder magnetic core include the iron loss and the magnetic flux density.
The iron loss is an energy loss generated in a magnetic substance when an alternating magnetic field is applied into the ferromagnetic substance. The electromagnetic component such as an inductor and a motor is often used in the alternating magnetic field and thus reduction in the iron loss of the powder magnetic core used for the electromagnetic component from the viewpoint of improvement of electromagnetic conversion characteristics.
The iron loss is represented by the sum of a hysteresis loss and an eddy current loss when the region does not include the relaxation phenomenon of the magnetic flux change in the material (such as magnetic resonance). The hysteresis loss is proportional to a driving frequency and the eddy current loss is proportional to the square of the driving frequency. Consequently, when the driving frequency is a high frequency (for example, more than 1 kHz), the effect of the eddy current loss on the iron loss is large, while when the driving frequency is low frequency (for example, several hundred Hz to 1 kHz), the effect of the hysteresis loss on the iron loss is large.
Among the electromagnetic components, the inductor and a reactor are used under a high drive frequency. As a result, reduction in the eddy current loss is important. It has been known that reduction in the eddy current loss can be achieved by covering the surfaces of iron based particles with insulating coating films. Generation of eddy current flowed across the particles can be suppressed by covering the surfaces of the iron based particles with the insulating coating films. By this treatment, the eddy current is localized in each particle and thus the eddy current loss can be reduced as a whole. As the insulating coating film, an insulating inorganic coating film (for example, a phosphate conversion coating film, a water glass coating film, and an oxide coating film) and a resin coating film (for example, a silicone resin coating film) are used. In order to reduce the eddy current loss, a soft magnetic powder having smaller particle size is also effective (for example, Patent Literature 1).
Among the electromagnetic components, the motor and the like are used under a low drive frequency. As a result, reduction in the hysteresis loss is important. It has been known that thermal treatment is recommended to be applied to a compact obtained by compacting the soft magnetic powder in order to reduce the hysteresis loss. In other words, the hysteresis loss has strong correlation to the coercive force. Therefore, as the strain introduced into the compact is increased, the coercive force of the powder magnetic core is increased. Consequently, the coercive force of the powder magnetic core becomes smaller when the thermal treatment (strain release annealing) is applied after compacting to release the introduced strain. As a result, the hysteresis loss of the powder magnetic core becomes smaller.
In order to increase the magnetic flux density, the magnetic flux density of the soft magnetic powder itself is required to be increased and thus a pure iron powder containing less impurity elements is preferable. The magnetic flux density can also be increased by increasing the compact density of the powder magnetic core.